1. Field of the Invention
The present invention relates to a shadow mask in a color cathode ray tube (CRT), of an Fe--Ni series invar alloy having uniform electron beam pass-through holes, and the method of preparation of the same which include adjusting the alloy composition, crystal grain size, and concentration of {100} crystal planes so as to have excellent etchability and formability for forming the uniform electron beam pass-through holes with less etching deviation and better roundness.
2. Discussion of the Related Art
Referring to FIG. 1, a color CRT is provided with a panel 1 coated with fluorescent films 3 on an inside surface thereof, a funnel 2 coated with conductive graphite on an inside surface thereof and fusion welded to the panel 1 with a glass at a temperature of approx. 450.degree. C. in a furnace, an electron gun 6 in a neck portion 4 of the funnel 2 for emitting electron beams 5, the shadow mask 7, being a color selection electrode, supported by frames 8 on an inner side of the panel 1, and deflection yokes 9 for deflecting the electron beams in left and right directions. The reference numeral 10 denotes an inner shield.
When a video signal is provided to the aforementioned color cathode ray tube, thermal electrons 5 are emitted from cathodes in the electron gun 6 and travel toward the panel 1, while being accelerated and focused by different electrodes in the electron gun 6. In the travel, the electron beams 5 are deflected causing changes in their travel path by a magnetic field generated by the deflection yokes 9 on the neck portion 4 of the funnel 2, thus scanning an entire surface of the panel 1. The deflected electron beams 5 are used to represent a color as they pass through a slot in the shadow mask 7 supported from an inside frame of the panel 1 since those electron beams collide with different fluorescent films 3 on the inside surface of the panel 1, to generate light, thereby reproducing the video signal.
A rimmed iron in JIS G3141 series or an aluminum killed steel(AK steel), each being a pure iron, are conventionally used as a material for fabricating shadow masks 7 in a color cathode ray tube. However, due to the large thermal expansion coefficients of these materials (pure steel: 11.5.times.10.sup.-6 deg.sup.-1) and the screen currently developed for a high definition TV, thermal expansion of the shadow mask 7 resulting from heat caused by collision of the electrons emitted from the electron gun and the shadow mask causes doming, which is a color dispersion experienced when the electron beams collide with a fluorescent surface corresponding to a color other than a designated color due to the thermal expansion. In order to prevent doming, an invar alloy of Fe--Ni series having a smaller thermal expansion coefficient (1.5.times.10.sup.-6 deg.sup.-1) is employed.
The shadow mask 7 is fabricated as follows.
A slab, formed from casting of a molten steel having an invar composition in a converter or an electric furnace, is subjected to hot rolling, annealing, acid cleaning and cold rolling, thereby forming a thin plate with a thickness of 0.1.about.0.5 mm. During cold rolling, the number of times the rolling is conducted depends on the reduction ratio. Then, an intermediate annealing process is conducted at a temperature over 800.degree. C., where the slab is temper rolled to control the thickness and surface roughness and annealed. The surface is cleaned and dried, a coat of photoresist is applied, exposed and developed, etched by a ferrous chloride solution, removed, cut, etc., to obtain a circular plate with holes. The circular plate is then cleaned, dried, annealed at a temperature over 800.degree. C., hot pressed, black iron oxide coated, weld assembled and packed, to obtain a shadow mask as shown in FIG. 1.
As the shadow mask of invar alloy has a small thermal expansion coefficient, facilitating to form an exact pass of the electron beams irrespective of the temperature, the invar alloy is widely used as a material of shadow masks suitable for display of high definition TV broadcasting systems and computers which require a high definition still image. In order to obtain a high definition shadow mask of such an invar alloy, small pitched uniform holes should be formed in the shadow mask material by etching. However, despite its low thermal expansion coefficient, the invar alloy is a material which has typically not etched well with difficulty in obtaining uniform holes. Therefore, improving the etchability of the invar alloy has been an important subject to be solved. For example, Japanese laid open patent No. S61-82453 restricts the carbon content to be below 0.01% and Japanese laid open patent No. S61-84356 restricts the non-metallic components, for improvement of the etchability. And, Japanese patent publication No. S59-32859, Japanese patent publication No. S61-19737, Korean patent publication No. 88-102 and 87-147, and U.S. Pat. No. 4, 528,246 claim that a shadow mask material of invar alloy with a concentration over 35% of {100} crystal planes obtained by controlling the cold rolling and annealing in the shadow mask raw material forming process permits good etching to facilitate the formation of uniform electron beam pass-through holes, resulting in a reduction in the doming, and thus improved color reproduction. However, conventional invar alloy material has S, B, N impurities even when the carbon content is below 0.01%. Since the impurities are segregated from crystal grains or exist as interstitial atoms in the crystal when annealed, the impurities affect the etchability. Thus, the impurities should be controlled. In view of the {100} crystal plane having the fastest etch rate, the etching can be carried out efficiently when the {100} planes are agglomerated in a rolled surface. However, if the {100} crystal plane concentration is too high, the fast etching results in the formation of non-round holes, particularly, if the concentration is over 95%. The etching proceeds along the crystal plane, resulting in the formation of holes which are not round and non-uniform. Therefore, the concentration of the {100} crystal planes over 35% as disclosed by Japanese patent publication No. S61-19737 claims is not satisfactory, since the etchability is influenced by other factors, such as crystal grain sizes, composition of elements, formation process conditions, orientations of the crystal grains, and the like or combinations thereof.